The present invention relates to a radiation-resistant optical fiber and to the applications thereof in various fields. More particularly, the invention relates to an improvement of the radiation resistance of a rare-earth-doped optical fiber, and in particular to the ionizing radiations. The invention also relates to the use of such a fiber in fiber amplifiers or lasers intended to be used in the presence of radiations.
Description of the Related Art
Optical fibers have known a very strong development due to their properties of high-rate transmission of information in very constraining environments (aerial, underground, submarine). Moreover, the immunity of optical fibers to electromagnetic radiations ensures the reliability of the transmissions. The so-called standard optical fibers have found applications in the long-haul terrestrial and intercontinental communications thanks to their low linear attenuation and their high stability. In the last twenty years, the rare-earth-doped optical fibers have been developed for applications in the optical fiber amplifier and in the fiber lasers.
However, optical fibers are not fully immunized with respect to their environment but are generally sensitive to some factors such as temperature, pressure or radiations such as ionizing radiations.
As used herein, the term “radiation” means an electromagnetic or corpuscular radiation capable of directly or indirectly producing ions, or of moving atoms during its passing through the atoms and the molecules of the matter. The radiation sources may be natural (cosmic radiation in space) or artificial: radiations of photons (X-rays, γ-rays), neutrons, protons, heavy ions present in nuclear facilities, in scientific instruments or in medical devices.
The use of optical fibers in radiative environment has a major interest in the civil, military or space fields, due to the numerous advantages of the fibers and in particular their electromagnetic immunity, their wide bandwidth, their light weight and low volume. The optical fibers can be used as tools for transporting information, as sensors or as diagnostic elements. The optical fibers used in nuclear environment may be passive or active, and in the latter case, they are generally doped and used as an active medium in amplification or laser mode.
However, it has been observed that exposing an optical fiber to radiations induces a degradation of the optical performance, in particular an increase of the linear attenuation of the fiber (referred to as the Radiation-Induced Attenuation or RIA).
Many studies have been done about various types of optical fibers so as to analyze the RIA effects, to determine the causes thereof and to find treatments for improving the resistance of the optical fibers to the radiations.
On the one hand, the radiation-induced attenuation depends on the type of radiation, the dose rate and the total dose received by an optical fiber. On the other hand, the radiation-induced attenuation strongly depends on the fiber composition and varies in particular according to whether it is a fiber with a core made of pure silica, of phosphorus-doped silica, or a rare-earth-doped optical fiber.
The patent U.S. Pat. No. 5,681,365 (Gilliland et al.) describes a method for improving the radiation resistance of a silica-core optical fiber comprising doping the core with fluorine and/or applying a tension lower than 5 grams at the time of fiber drawing. The fiber is then exposed to one of the two following types of radiation sources: a γ-ray source (60Co) having dose rate of ˜100 rad/s for a total dose of 1.44-1.7 krad (14.4-17 Gy) or a K-alpha copper source of 250 kV producing a total dose of 3.8 krad (38 Gy). According to the patent U.S. Pat. No. 5,681,365, the positive impact of fluorine doping and low-tension fiber drawing and the synergy between these two factors allow reducing the RIA at the measurement wavelength of 1300 nm.
The patent U.S. Pat. No. 4,988,162 (Hayami) describes a radiation-resistant multi-core optical fiber, each of the core being made of pure silica. According to this patent U.S. Pat. No. 4,988,162 (Hayami), the reduction, in the silica core, of the chlorine impurities to OH groups and fluorine allows reducing the RIA in the visible domain when the fiber is exposed to a γ-ray source (60Co) having a dose rate of 2.104 rad/h (210 Gy/h) and for a total dose of 3.105 (3.1 kGy/h).
The patent U.S. Pat. No. 5,574,820 (Griscom) describes a method for radiation-hardening a pure-silica-core optical fiber. The method comprises pre-irradiating the fiber with a radiation source (γ or fast neutrons) at a dose of at least 107 rad (i.e. 105 Gy) to induce a permanent attenuation in the fiber so that a subsequent exposure to a dose of radiations induces only a limited RIA at 30 dB/km at most in the visible domain (400-700 nm). The patent U.S. Pat. No. 5,267,343 (Lyons et al.) describes another method for hardening an ultrapure-silica-core fiber that combines hydrogenating, then pre-irradiating the fiber by a γ-ray source (60Co), at a total dose going up to 106 rad (104 Gy). According to U.S. Pat. No. 5,267,343, this method allows lengthening the lifetime of the fibers and also provides a protection in the UV range. However, the hydrogenation creates an absorption band in the amplification band around 1.5 μm and/or in the pumping band.
The publication of D. L. Griscom, E. J. Friebele, K. J. Long, and J. W. Fleming, “Fundamental defect centers in glass: Electron spin resonance and optical absorption studies of irradiated phosphorus-doped silica glass and optical fibers”, J. Appl. Phys. 54, 3743 (1983), analyses the peaks of optical absorption induced in phosphosilicate fibers by ionizing radiations (γ source (60Co) or X-rays of 56-100 keV). The defect of type P1 is correlated to an absorption induced at 0.8 eV (˜1550 nm).
Likewise, the publication of S. Girard, J. Keurinck, A. Boukenter, J-P. Meunier, Y. Ouerdane, B. Azaïs, P. Charre and M. Vié, Nucl. Instr. and Methods in Physics Res. Sec. B, Vol. 215, n° 1-2, p. 187-195, 2004, compares the RIA in various types of germanium-, nitrogen-doped and phosphorus-codoped, pure-silica-core fibers. According to this study, the phosphorus codoping is responsible for the highest permanent RIA levels after irradiation either by a γ-ray source or by a pulsed X-ray source. The predominant influence of phosphorus on the RIA is attributed to the formation of permanent phosphorus coloured centers of the POHC (Phosphorus Oxygen Hole Center) type, and P1 having an absorption band around 1600 nm.
The strong radiation sensitivity of the phosphorus-doped silica-core fibers has been advantageously used for making radiation dosimeters. Hence, the publication M. C. Paul, D. Bohra, A. Dhar, R. Sen, P. K. Bhatnagar, K. Dasgupta, “Radiation response behavior of high phosphorous doped step-index multimode optical fibers under low dose gamma irradiation”, Journal of Non-Crystalline Solids 355 p. 1496-1507 (2009), shows that a phosphorus-doped fiber has, in the domain of 500-600 nm, an almost-linear absorption as a function to the radiation dose of a source 60Co for low doses comprised between 0.1 and 100 rad (0.001 and 1.0 Gy). Moreover, the sensitivity of the fibers increases with the phosphorus rate.
The publication H. Henschel, M. K. Orferb, J. Kuhnhenn, U. Weinand, F. Wulf, Nuclear Instruments and Methods in Physics Research A 526 (2004) 537-550, describes the use of a phosphorus-doped optical fiber as a radiation sensor in a particle accelerator by a reflectometry measurement in the visible (670, 865 nm) or the near IR (1300 nm). The optical time-domain reflectometry measurement (OTDR) makes it possible to obtain a dosimetry system distributed along the accelerator. Moreover, the publication Henschel et al. indicates that the rare-earth-doped active optical fibers are extremely sensitive to the ionizing radiations.
Among the active fibers, the erbium-doped fiber is of major interest. Indeed, the erbium-doped fibers allow the making of amplifiers or lasers with an emission wavelength located around 1500 nm, commonly referred to as the third telecom band. The erbium-doped fibers also enter in the making of so-called “ASE” wideband sources, of communication amplifiers of the EDFA type, and of fibered lasers, which are used for example for free space communication applications or for making LIDARs.
Most often, the core of an erbium-doped silica fiber is codoped with germanium, or even with aluminum. A particular branch of the erbium-doped fibers is the branch of the erbium-ytterbium-codoped fibers. Theses fibers are an alternative to the simply erbium-doped fibers, more particularly for high-power applications. FIG. 1 shows a sectional view of a double-cladding optical fiber. The optical fiber 5 comprises a rare-earth-doped core 1, surrounded by a first pump-guiding cladding 2, surrounded by a second cladding 3. The fiber 5 comprises an outer protective coating 4.
FIG. 2 shows the various levels of energy of the ytterbium and erbium ions in an erbium-ytterbium-codoped material. In an erbium-ytterbium-codoped fiber, the erbium ion is not excited directly by a 976-nm single-mode pump on the level 4I11/2 of the erbium, but is excited indirectly via single mode pumping in the absorption band between 900 and 980 nm of the ytterbium ions, i.e. on their level 2F7/2→2F5/2. The ytterbium ions then transfer their energy to the erbium ions via transfers of the phonon type. Now, the phonon energy of the link P═O (phosphorus-oxygen) is ideally adapted to this energy transfer. To improve the energy transfer between the ytterbium and the erbium, the core of an erbium-ytterbium-codoped silica optical fiber is made in a conventional way with phosphorus doping. Phosphorus is then used both to increase the refractive index of the core and to fulfill the function of energy transfer between the rare earth elements. The use of a phosphosilicate matrix in the fiber core, rather than a pure-silica matrix, has also for indirect effect to increase the lifetime of the level 2F5/2 of the ion Yb3+, which limits the emission at 1 μm of this rare earth, and to reduce the lifetime of the level 4I13/2 of the Er3+, which limits the phenomenon of reverse transfer.
However, as mentioned hereinabove, the phosphorus is a dopant of the silica known to have a strong impact on the radiation resistance of the silica-based optical fibers. Indeed, the presence of phosphorus induces a strong radiation sensitivity of the optical fiber, which results in an excessively high level of the attenuation induced during irradiation and in an absence of recovery of the transmission capacities of the optical fiber after irradiation, contrary to most of the other types of optical fibers. This strong sensitivity is particularly marked in the infrared due to the generation of a defect relating to the phosphorus, the center P1 that has an absorption band centered around 1600 nm, very close to the third telecom window. Moreover, the defect concentration increases even after the end of irradiation due to the thermal conversion of other defects into defects P1.
Accordingly, the phosphorus element is generally banished both in the core and in the optical cladding of any rare-earth-doped optical fiber intended to be used as a laser source or an optical amplifier in a radiative environment. The phosphosilicate-matrix, erbium-ytterbium-codoped, double-cladding fibers are thus commonly prohibited in a radiative medium, their strong radiation sensitivity in the third telecom band (around 1.5 μm) being mainly associated to the presence of phosphorus defects.
This radiation sensitivity of the rare-earth- and/or phosphorus-doped optical fibers prevents from using the fibers for applications of laser transmission, amplification or emission in the mediums where the ionizing radiations are naturally present (for example, in space) or produced (nuclear reactor environment). Indeed, the performance of the fiber amplifiers or the fiber lasers is liable to degrade very rapidly as a function of the received radiation dose.
The radiation-induced attenuation is only partially reversible over time, by thermal-bleaching or photo-bleaching processes. However, in certain environments, as in the spatial field, the radiations are permanent and the bleaching processes are impracticable.
A lot of works have been done for increasing the radiation resistance of optical fibers. As used herein, the phrase “radiation resistance”, applied to an optical fiber, means the resistance of a fiber to an increase of its linear attenuation when exposed to radiations such as X-rays, neutrons, gamma-rays or others. Generally, the radiation resistance may be measured by the attenuation induced by the exposure to the radiations, by the recovering rate, or by the time required for a partial or total recovery.
There exists a need for fiber amplifiers or lasers operating in the third telecom band (˜1.5 μm) and being radiation-resistant, i.e. having a low radiation-induced attenuation. It is for example desirable that optical amplifiers or fiber lasers can operate in optical systems installed in a nuclear or spatial environment. Therefore, there exists a need to make a radiation-resistant rare-earth-doped optical fiber.